As an alternative to conventional fixed frequency power generation systems, recent aerospace applications have utilized variable frequency generators that typically deliver electrical power at frequencies between 320 and 800 Hz. In one such application, a synchronous machine (e.g., a brushless synchronous machine) operates in a first mode as a generator to convert mechanical energy from a prime mover, such as a gas turbine engine, into variable frequency AC power and operates in a second mode as a main or auxiliary power unit starter to convert electrical power into mechanical power, which is supplied to the prime mover until it reaches a self-sustaining speed.
Traditionally, an auxiliary power unit (APU) start power system architecture has two main subsystems: a start power unit (SPU); and a start converter unit (SCU). FIG. 1 illustrates a conventional aircraft power system 10, which includes: a synchronous machine 20; an SCU 30; and an SPU 40. The SCU 20 includes a first inverter 32, which supplies multi-phase AC power to the synchronous machine 20 during a starter mode. The SCU 30 includes a second inverter 34 to provide the exciter power supply field for the synchronous machine 20.
The SPU 40 includes an AC to DC converter 42 and a DC to DC converter 44. When connected to an AC input power source during the starter mode, the AC-DC converter 42 converts AC power from the AC input power source (e.g., supplying 115 VAC) to a DC bus/link voltage, typically around 270 VDC or higher. In FIG. 1, the AC input power source is illustrated as an aircraft start ground panel 64. When connected to an on-board DC power source (battery) 62 or a ground-based DC power source (aircraft start ground panel 64) during the starter mode, the DC—DC converter 44 converts input DC power to the DC bus voltage. The DC input power is typically 28 VDC. During starter mode, the first inverter 32 converts DC power from the SPU 40 to a three-phase voltage for the stator windings of the synchronous machine 20. Furthermore, the second inverter 34 converts the DC power from the SPU 40 to an Exciter Power Supply (EXPS) voltage. The first inverter 32, the second inverter 34, the AC-DC converter 42, and the DC—DC converter 44 are typically provided as discrete units (“boxes”), thereby each requiring a separate DC link. Although not shown, the first inverter 32, the second inverter 34, and the DC—DC converter 44 include individual controllers in this conventional arrangement. In this arrangement, DC bus regulation is performed by a generator control unit (GCU, not shown), through a point of regulation either at the input three-phase AC power or at the DC bus.
During generator mode, the AC-DC converter 42 of the SPU 40 converts multi-phase AC power from the synchronous machine 20 into a DC bus voltage. The AC-DC converter 42 performs passive rectification and is comprised of a three-phase diode bridge in this conventional arrangement. The first inverter 32 of the SCU 30 converts the DC bus voltage to variable voltage (VV), fixed frequency (FF) or variable frequency (VF) power to supply aircraft load(s).
As illustrated, power flow in the conventional arrangement of FIG. 1 is not bi-directional and is managed using at least the following electrical contactors: a first contactor 50-1 between the synchronous machine 20 and the AC side of the first inverter 32; a second contactor 50-2 between the synchronous machine 20 and the AC side of the AC-DC converter 42; a third contactor 50-3 between the first inverter 32 and aircraft load(s); a fourth contactor 50-4 between external DC power source 64 and the DC—DC converter 44; a fifth contactor 50-5 between the external AC power source 64 and the AC side of the AC-DC converter 42; and a sixth contactor 50-6 between aircraft battery 62 and the DC—DC converter 44. Opened/closed states for these various electrical contactors to achieve different operating modes are shown below in Table 1, wherein “C” represents a closed contactor state and “O” represents an opened contactor state.
TABLE 1Different Modes of OperationStarter/Gen. Modes50-150-250-350-450-550-6SM Start Ground DCCOOCOOSM Start Aircraft DCCOOOOCSM Start Ground ACCOOOCOSM GenerateOCCOOO
This particular arrangement of electrical contactors is required in part due to uni-directional power flow requirements. Furthermore, thermal management and DC linking for the SCU 30 and the SPU 40 are fragmented. This is illustrated in FIG. 1 as separate heat sinks and DC links for SCU and SPU components (heat flow being represented as cross-hatched arrows). Separate connectors (power and discrete input/output) are typically required. Thus, overall system weight, volume, and cost are increased and reliability is reduced due to increased part count.